This invention relates to spark ignited internal combustion engines including apparatus for sensing knock and capable of controlling engine spark timing or some other engine operating parameter to limit such knock to acceptable levels. There has been increasing interest in recent years in engines with such knock sensing and controlling apparatus due to such factors as the lowering of octane in fuels in order to obtain the maximum amount of fuel from crude oil, the elimination of lead compounds from the fuel, the desire to operate engines on the borderline of knock for maximum engine efficiency, the increased use of turbochargers and the desire to produce engines with higher compression ratios and lighter weight for greater power and fuel economy.
The usual knock sensing apparatus includes a vibration or knock sensor fixed to a surface of the engine to sense the vibrations of the engine itself and generate an electrical output signal which may be amplified and processed to improve the signal to noise ratio and then applied to appropriate control apparatus. With respect to engine vibrations, however, the typical spark ignited internal combustion engine is a very noisy environment. Engine vibrations from sources other than knock, such as valve train clatter, piston slap, etc., are often present in amplitude equal to or greater than those vibrations due to knock and can be easily mistaken for knock vibrations by the sensing apparatus.
As a result, most such systems utilize some form of frequency discrimination to take advantage of the fact that knock vibrations appear, in most engines, to occur at a specific frequency for a particular engine construction. Although this is helpful to some extent, it does not fully solve the problem. The knock frequency cannot be fixed too exactly, since it may vary slightly from one engine to the next and even from one cylinder to the next within a specific engine. In addition, many of the engine vibrations due to sources other than knock also may appear at times to have a substantial output within the range of the characteristic knock frequency. Thus, even a well designed system may have some response to spurious engine vibrations incorrectly interpreted as knock with a resultant "false retard", that is, a retard of spark timing with resultant lowering of engine efficiency when no such retard is actually required. Straightforward methods of reducing the tendency of the system to respond to the spurious vibrations, such as analyzing their source and eliminating them or locating the knock sensor to minimize the magnitude of such vibrations at the sensor site have proved to be extremely difficult and generally unsatisfactory.
Research appears to indicate that the characteristic vibrations due to knock involve the excitation of acoustic cavity resonances in the combustion chambers of the engine. Detonations cause the cavity to resonate and thus excite the engine structure. The so-called knock frequency corresponds to the frequency of the first cavity mode of resonance. In practice, this first mode occurs at approximately the same frequency in all cylinders and may vary somehwat with different engine operating conditions. The knock frequency can be determined from a spectral analysis of the combustion chamber pressure when an engine is knocking. Actually, a typical combustion chamber spectrum will produce a time-averaged spectrum that reveals several identifiable peaks; and a time-averaged spectrum of the output of a properly placed, engine mounted accelerometer will show peaks at the same frequencies.
However, for a single cylinder firing event, a spurious vibration may occur at one of these cavity resonance frequencies. Therefore, no matter which of the frequencies is selected as the "knock frequency", there may be some proportion of the detected "knock events" which are spurious and cause "false retard".